The present invention generally relates to telecommunication test systems and methods, and more particularly to telecommunication tests and methods that allow transmission systems, such as Subscriber Line Multiplexer Systems to effectively respond to automatic test procedures, such as the MLT (Mechanized Loop Test).
Transmission systems have been used to provide cost-effective connections between a central office and subscribers' telephones. A Subscriber Line Multiplexer system is a type of transmission system that uses digitally multiplexed channels to connect two or more telephone subscribers to a central office. This system uses the ISDN U-interface technology, or xDSL technologies, such as HDSL (High Data Rate Subscriber Line), ADSL (Asymmetric Digital Subscriber Line) and their variations, to multiplex several voice and/or data channels over a single copper pair, in order to provide additional virtual telephone or data lines to one or more subscribers over the existing cable facilities.
When the personnel in the central office of a telephone company receives a complaint from a subscriber about telephone problems, the subscriber's line is remotely tested from the central office to verify the accuracy of the complaint and to locate defective network components. Thus, a repairing team may be dispatched to the appropriate location. This provides an efficient and economic solution for the telephone company in dealing with problems about telephone lines.
Typically, the telephone line tests are automated, and one of the most popular automated tests is known as the MLT procedure performed by an MLT system. In an MLT procedure, the MLT system applies known electrical signals on the subscriber's loop comprising the telephone line and the subscriber's telephone set and makes appropriate measurements. Based on the measurements, the MLT system can calculate the loop impedance, the parasitic impedances, as well as possible foreign voltages (i.e., voltages other than those supplied from the central office) present on the line. The MLT system reports back to the central office the results of the measurements, as well as the probable status of the line. The operator needs to know these results in order to understand the problems and take appropriate actions. For example, if the insulation between the wires of a telephone line is damaged, the impedance between wires may become very low. In this case, the transmission characteristics of the telephone line may become very poor, and the quality of the services could thus be impaired. Also, the presence of a foreign voltage due to, e.g., induction from defective high power electrical ducts, can impair the transmission characteristics of the telephone line and can even make it dangerous to use the telephone.
The MLT procedure is most effective for remote testing telephone lines without having any transmission system, such as a fiber optic transmission system, or a Subscriber Line Multiplexer System connected to the lines. If the subscriber's telephone set is not directly connected to the central office, but instead it is connected through a transmission system, the MLT system does not have direct access to the line. In such a case, when the transmission system detects the presence of an MLT signal, it feeds back to the MLT system the values of three resistors located at the subscriber's loop which represent the "signature" of the current state of the system. These three resistors are respectively connected between the ring and tip lines, the ring line and ground and the tip line and ground.
In order to present the proper signature to the MLT system, the transmission system must perform a series of self tests. If the number of lines serviced by the system is large, the remote equipment can be provided with a rather complex test head for performing elaborate tests of the subscriber's drop (i.e., the copper pair that connects the remote terminal to the subscriber). The test head can report back the results to the central office terminal for presenting the proper signatures. Because large number of lines are serviced, the per-line cost of the test head is relatively small. However, in the case of multiplexer systems with small number of lines, e.g., 2 or 4 lines, the per-line cost of the test head is very high. One solution would be to use a less sophisticated test head with simple self-test circuits. In such case, the system periodically performs (or at a time when the MLT is detected) simple tests which will establish signatures for presenting to the MLT system. However, the number of tests and the accuracy of the results obtained with these simple circuits are insufficient for detecting the problems of the telephone lines. Thus, complex circuits have to be used, which dramatically increases the cost.
Accordingly, there is a need to provide an improved and cost-effective system and method for testing telephone lines connected to transmission systems, such as Subscriber Line Multiplexer systems.